VCE Chemistry Question Thread

[Eiffel]

with significant figures i know all of them however i have a problem. If my original question has say 4 parts with say 6 different numerical values, and in the intro of the Q it has 2 SF as lowest amount of SF, and then when doing part A it only requires 4 of the 6 parts and the lowest SF out of the 4 is 3SF. Do i express as 2 or 3 SF?

[Eiffel]

For question 2, you were right up until n(hcl). What you would do then is:
n(Al)= (2/6)*0.05= 1/60 mol. Then use the mole ratio to find the mol of H2; n(H2)= n(Al)* 3/2= 0.0250.
Finally v(H2)= 0.0250*22.5=0.562

I see, i didnt read that it said Hcl even though i wrote it out, treated it as H2 the whole time lol. Thanks very much, i see my mistake.

[keltingmeith]

with significant figures i know all of them however i have a problem. If my original question has say 4 parts with say 6 different numerical values, and in the intro of the Q it has 2 SF as lowest amount of SF, and then when doing part A it only requires 4 of the 6 parts and the lowest SF out of the 4 is 3SF. Do i express as 2 or 3 SF?

This is an incredibly confusing example... However, you use the lowest amount of sig figs in the calculation you're doing. So, if you do 0.200/0.400, even though 0.3 is listed somewhere, you'd still write 0.500 as your answer - because only the 0.200 and 0.400 are used in your calculations.

'covalent bonds tend to be stronger than ionic bonds but this means that, to break these bonds, help is needed in the form of special proteins called enzymes' ?

Sorry about that, strong is just a very... "Not specific" word. :S

Honestly, the textbook is being incredibly wishy-washy about all of this. Enzymes catalyse reactions by reducing the amount of activation energy required (like any catalyst) - whether something is held together by ionic or covalent bonds, it still needs to pass this activation energy. As far as I'm concerned, the fact that's a covalent compound instead of an ionic compound is pretty irrelevant... Where is this mentioned?

And for the water question (because I wasn't sure if it was related to the first one or not), NaCl dissassociates in water because it is a polar molecule. Water is also a polar molecule, so the polarity of the water interferes with the polarity of the salt, and it'll dissociate in the water.

[Cogglesnatch Cuttlefish]

I see, i didnt read that it said Hcl even though i wrote it out, treated it as H2 the whole time lol. Thanks very much, i see my mistake.

Yeah happens to me too man. It's bewildering; you're sure you're taking the necessary steps and all and the answer just doesn't add up.

[Cogglesnatch Cuttlefish]

From the question that Eiffel posted, 2Al + 6HCl ------> 2AlCl3 + 3H2, would the mole ratio between HCl and H2 be 2:1 still? 

[keltingmeith]

From the question that Eiffel posted, 2Al + 6HCl ------> 2AlCl3 + 3H2, would the mole ratio between HCl and H2 be 2:1 still?

Yes.

6:3
Divide both sides by 3,
2:1

[Cogglesnatch Cuttlefish]

Yes.

6:3
Divide both sides by 3,
2:1

Doesn't the fact that HCl has 1 mol of hydrogen/ mol of HCl and H2 has 2 mol of hydrogen/ mol of hydrogen gas affect the ratio at all?

[keltingmeith]

Doesn't the fact that HCl has 1 mol of hydrogen/ mol of HCl and H2 has 2 mol of hydrogen/ mol of hydrogen gas affect the ratio at all?

It depends on what you're trying to find, here.

For ever two moles of HCl, there is one mole of H2.
For every one mole of H2, there are two moles of hydrogen.

So, for every two moles of HCl, there are two moles of hydrogen. However, there is still one mole of H2 for every two moles of HCl.

[Cogglesnatch Cuttlefish]

It depends on what you're trying to find, here.

For ever two moles of HCl, there is one mole of H2.
For every one mole of H2, there are two moles of hydrogen.

So, for every two moles of HCl, there are two moles of hydrogen. However, there is still one mole of H2 for every two moles of HCl.

Oh right. Thanks 

[Maths Forever]

can someone explain 2 and 3

2) v(hcl) = 0.5L , c(hcl) = 10^-1 hence n(hcl) = 0.05 sub into n = v/v(stp) gets 1.12?

3) i dont know how this is done

Hi Eiffel,

For question 2, like you said, C (HCl) = 0.1 M (if the pH = 1)

Hence n (HCl) = C x V = 0.1 x 0.500 = 0.05 mol

Now, the reaction between Al and HCl will be as follows:

2 Al (s) + 6 HCl (aq) --> 3 H₂ (g) + 2 AlCl₃ (fully balanced)

Hence n (H₂) = 1/2 n (HCl) = 1/2 x 0.05 = 0.025 mol

At STP, molar volume (Vm) = 22.5 L / mol

Vm = V (H₂) / n (H₂)

Therefore V (H₂) = n (H₂) x Vm = 0.025 x 22.5 = 0.56 L

Which is option B.


Now for Question 3, you need to figure out the mole ratio between the unknown acid and sodium carbonate.

n (Na₂CO₃) = m / M = 15.9 / [(23.0 x 2) + 12.0 + (16.0 x 3)] = 15.9 / 106 = 0.150 mol

n (Unknown Acid) = C x V = 2.00 x 0.150 = 0.300 mol

Hence the ratio of n (Na₂CO₃) : n (Unknown Acid), is equal to 0.150 : 0.300, which simplifies to 1:2.

If the acid is monoprotic, it can donate one hydrogen atom. If diprotic, two hydrogen atoms. If triprotic, three hydrogen atoms.

The general unbalanced reaction equation will be Na₂CO₃ + H_xZ --> H₂O + CO₂ + Na_yZ, where Z is a anion such as sulfate (SO4^2-) or chloride (Cl^-).

Now we know the ratio between (Na₂CO₃) and Unknown Acid (or H_xZ) is 1:2.

So the first step to balancing the equation is this:

1) Na₂CO₃ + 2 H_xZ --> H₂O + CO₂ + Na_yZ

The oxygen atoms are already balanced (3 atoms on both sides of the equation). So the ratio between Na₂CO₃, H₂O and CO₂ must be 1 : 1 : 1.

Since Z must also be balanced, the ratio between H_xZ and Na_yZ must be 2 : 1 (as seen in step 1).

So therefore the value of 'y' must be 2, so that the sodium atoms will balance.

2) Na₂CO₃ + 2 H_xZ --> H₂O + CO₂ + Na₂Z

The last step is to balance the hydrogen atoms. We have 2 'x' hydrogen atoms on the left hand side, and 2 hydrogen atoms on the right hand side.

Hence, 2x = 2. Therefore x = 1. So the last step is to substitute this 'x' value into the equation.

3) Na₂CO₃ + 2 H₁Z --> H₂O + CO₂ + Na₂Z

Which is written as Na₂CO₃ + 2 HZ --> H₂O + CO₂ + Na₂Z.

So since HZ has one hydrogen atom to donate, the unknown acid must be 'monoprotic'.

Which is option A.

A long explanation I know. But it is difficult to describe. Let me know if there are any uncertain spots.

Thanks

[Chang Feng]

Hello I was just wondering if I can get some conceptual understand help. Why is concentration mol/l and density gram/l. What exactly is molar volume and how it relates to density. Since molar volume is equal to RT/density. Thanks

[lzxnl]

Hello I was just wondering if I can get some conceptual understand help. Why is concentration mol/l and density gram/l. What exactly is molar volume and how it relates to density. Since molar volume is equal to RT/density. Thanks

The concentration you've given us is the 'molar concentration', in other words the number of moles per litre. It's just one type of concentration that happens to be useful in chemistry.
Again, the density you've given us is a specific type; this is the mass density over a volume. For a thin circular disk, its density would be more appropriately measured using its mass divided by its area, whereas you can measure other densities like population densities.

As for the molar volume, it's defined to be the volume of one mole of said substance. This is V/n, which for an ideal gas is also RT/P. It's not directly related to the density as mass densities relate masses and not molar quantities.

[RazzMeTazz]

What actually is the difference between a physical and chemical property?

I know this is junior science stuff but I'm not sure..

[EspoirTron]

Physical property is displayed when there is no change in composition, e.g. mass, density, colour or volume changes. A Chemical property is one that allows a substance to undergo some change in composition, e.g. strong acid + strong base into a salt and water.

[stockstamp]

Hi, I need some help with this question please

What volume of 0.100 M sulfuric acid would be required to neutralise a solution containing 0.500 g of sodium hydroxide and 0.800 g of potassium hydroxide?

I'm able to get the correct answer. Any help would be appreciated